Answer the question on basis of the passage given below:
In zwitter ionic form, amino acids show amphoteric behaviour as they react both with acids and bases.
Except glycine, all other naturally occurring \(\alpha \)- amino acids are optically active, since the \(\alpha \)-carbon atom is asymmetric. These exist both in 'D' and 'L' forms. Most naturally occurring amino acids have L-configuration. L-amino acids are represented by writing the \(NH_2\) group on the left hand side.
Identify the amino acid which is amphoteric in nature:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → NH2–CH2–COOH (glycine)
Note: The given answer is as per the official NTA Answer Key. However, there is more than one correct option for this question, as explained below.
Every α-amino acid has an acidic –COOH group and a basic –\( \ce{NH2} \) group. Amino acids are classed as acidic, basic or neutral by comparing the number of amino and carboxyl groups in the molecule. Glycine has one –NH2 and one –COOH, so its acidic and basic character balance; in its zwitter ion form, \( \ce{H3N^+-CH2-COO^-} \), it reacts with both acids and bases.
Option (1) Arginine — Is also correct. Its side chain carries an extra basic guanidino group, so it is classed as a basic amino acid. All the same, it has an –\( \ce{NH2} \) group and a –COOH group, so in its zwitter ion form it reacts with both acids and bases and is amphoteric, as the book says of amino acids.
Option (2) Histidine — Is also correct. Its side chain carries a basic imidazole ring, so it is classed as a basic amino acid. All the same, it has an –\( \ce{NH2} \) group and a –COOH group, so in its zwitter ion form it reacts with both acids and bases and is amphoteric, as the book says of amino acids.
Option (3) Aspartic acid — Is also correct. It has two –COOH groups and only one –\( \ce{NH2} \), so it is classed as an acidic amino acid. All the same, it has an –\( \ce{NH2} \) group and a –COOH group, so in its zwitter ion form it reacts with both acids and bases and is amphoteric, as the book says of amino acids.
NCERT (2026-27 edition):
Classification of Amino Acids – “… Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. Equal number of amino and carboxyl groups makes it neutral; more number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic.”
Classification of Amino Acids – “… In zwitter ionic form, amino acids show amphoteric behaviour as they react both with acids and bases.”